Some of Britain's biggest charities are seeking urgent legal advice over new lobbying rules which they fear could silence them on vital issues, it has been reported.

Oxfam, the Salvation Army and Shelter believe Government plans to tighten rules during election campaigns to block undue influence could have "disastrous unintended consequences", The Times said.

In a letter to Cabinet Office Minister Chloe Smith, seen by the newspaper, the charities warned that organisations could be frightened of speaking out on issues of major public concern in case they breach the new regulations.

"We are concerned that proposed new rules could apply to a range of normal and legitimate awareness-raising activities despite them being intended to be party-politically neutral," the letter said. "A health charity could publish a leaflet highlighting the dangers of smoking. If smoking legislation became a party political issue in an election this activity could be deemed to have the effect of supporting a party's campaign and be subject to regulation."

The measures, which also include a spending limit on any organisation promoting a party political message, are included in the Lobbying Bill.

A Cabinet Office spokeswoman insisted that charities campaigning without any intention to promote a particular party would be exempt from the rules. But Sir Stuart Etherington, chief executive of the National Council for Voluntary Organisations, said the Bill needed completely redrafting.

The letter added that the vagueness of the rules could "mute" charities on issues that matter to the public.

"The intention of the Bill is to bring greater transparency where third parties campaign in a way which supports a particular political party or its candidates, by requiring expenditure on those campaigns to be fully recorded and disclosed, " the Cabinet Office spokeswoman said."This Bill does not include campaigning by third parties - charities or other organisations - that is not intended to promote or could not reasonably be considered to promote the electoral success of any particular party. So a third party campaigning only on policy issues... would be exempt."

TUC general secretary Frances O'Grady said: "More and more people are waking up to this chilling attack on free speech.

"Any organisation that expresses a view that a political party disagrees with - even an extremist one - could be hit by this Bill. You can see why Government might like to suppress debate in the 12 months before an election but democracy suffers if people can't campaign on controversial issues such as HS2, fracking or NHS changes. This is why the Government should withdraw this Bill and start again - this time with some consultation."